Surface-passivated silicon quantum dot phosphors

ABSTRACT

Phosphors formed using silicon nanoparticles are provided. The phosphors exhibit bright fluorescence and high quantum yield, making them ideal for lighting applications. Methods for making the silicon phosphors are also provided, along with lighting devices that incorporate the silicon phosphors.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Patent Application No.61/564,947, filed Nov. 30, 2011, the disclosure of which is herebyincorporated by reference in its entirety.

STATEMENT OF GOVERNMENT LICENSE RIGHTS

This invention was made with Government support under contract DMR1035196 awarded by the National Science Foundation. The Government hascertain rights in the invention.

BACKGROUND

Semiconductor quantum dots (QDs) with size-tunable band gaps, highphotoluminescence (PL) quantum efficiency (QE), and high color purityhave shown a great potential for next-generation lighting and displays.Light-emitting devices embodied by QDs have two general forms: First,hybrid QD-organic light-emitting diodes (LEDs), which utilize QDs as theelectroluminescent layer while the organic semiconductor layers areresponsible for electron-hole injection. Second, light-converting LEDs,in which QDs are excited by wide band gap LEDs and emit the desired PLin longer wavelengths. Recently, using a state-of-the-art white QD-LEDbacklight system composed of InGaN blue LEDs and multiply-passivatedgreen- and red-light-emitting QDs as light converters, a highperformance LCD panel was successfully demonstrated for the first time.However, it is worthy of notice that previous QD-light-emitting researchwas predominantly based on group II-VI semiconductor QDs, such as CdSe,CdZnSe or CdZnS cores with single or multiple shells. Although suchcomplex and exquisite hetero-structures often lead to outstandingspecifications, for instance almost 100% PL QE, however, the highsynthesis cost and the toxicity from their heavy-metal ingredients mightshadow their potential for large-scale production and wide-spreadcommercialization.

Group IV silicon QDs (SiQDs), on the other hand, have gradually receivedmore attention, owing to their heavy-metal-free composition, chemicalstability and abundant starting materials. Recently, hybrid SiQD-organicLEDs have demonstrated electroluminescence from infra-red (IR) tovisible wavelengths. Extensive works have been contributed to thesynthesis of SiQDs. To date, main strategies include solution-basedprecursor reduction, heat-, laser- or plasma-induced aerosoldecomposition of SiH₄, thermal processing of sol-gel polymers derivedfrom HSiCl₃ and harvesting from nano-porous silicon. Except the lastone, all the other methods inevitably require critical conditions,special equipment or complex chemical reactions, all of which make themhard to achieve cost-down and scale-up. In contrast, porous silicon canbe easily prepared by electrochemical etching in a mixture of commonchemicals under ambient condition. The subsequent physical harvestingcan effectively separate the SiQDs from the silicon substrate.Noticeably, these highly luminescent powders mostly comprise micro-sizesilicon pieces with PL-emitting nanocrystal SiQDs trapped on theirsurfaces, rather than free-standing SiQDs.

Despite recent advances, if silicon-based phosphors are to be used inlighting devices, improvements in the phosphor materials themselves, aswell as the methods for making the phosphors, must be achieved.

SUMMARY

This summary is provided to introduce a selection of concepts in asimplified form that are further described below in the DetailedDescription. This summary is not intended to identify key features ofthe claimed subject matter, nor is it intended to be used as an aid indetermining the scope of the claimed subject matter.

In one aspect, a silicon phosphor is provided. In one embodiment, thesilicon phosphor includes:

a core comprising a silicon particle;

a plurality of silicon nanoparticles attached to the core;

a silicon oxide layer substantially encapsulating each of the pluralityof silicon nanoparticles; and

a passivating layer comprising a plurality of passivating ligands boundto the silicon oxide layer.

In another aspect, a method of making silicon phosphors is provided. Inone embodiment, the method comprises the steps of:

(a) electrochemically etching silicon to provide a plurality of coresilicon particles each having a plurality of silicon nanoparticlesattached thereto;

(b) isotropically etching the plurality of silicon nanoparticles toprovide a plurality of etched silicon nanoparticles;

(c) capping the plurality of etched silicon nanoparticles with an oxidelayer; and

(d) passivating the oxide layer with surface ligands.

In another aspect, a lighting device is provided that incorporates asilicon phosphor as described herein.

DESCRIPTION OF THE DRAWINGS

The foregoing aspects and many of the attendant advantages of thisinvention will become more readily appreciated as the same become betterunderstood by reference to the following detailed description, whentaken in conjunction with the accompanying drawings, wherein:

FIGS. 1A and 1B are diagrammatic illustrations of silicon phosphors inaccordance with the disclosed embodiments.

FIG. 1C is a flow chart illustrating a method for making siliconphosphors in accordance with the disclosed embodiments.

FIGS. 2A-2C. (2A) The normalized PL spectra of near-IR-, red-, orange-,yellow-, yellow-green-, and green-light-emitting colloidal SiQD-basedphosphor materials in ethanol. (2B) The normalized excitation spectra ofthe red-, orange-, yellow-, and yellow-green-light-emitting phosphorsmeasured in (2A). (2C) The PL spectra and EQEs of the red-light-emittingphosphors passivated with hydrides, hydroxyl groups and TMPS.

FIGS. 3A-3C. (3A) and (3B) The SEM images of a drop-casted phosphor thinfilm by low (103×) and high (105×) magnification, respectively. (3C)“UW” painted with the red-light-emitting phosphors on a glass slide.

FIGS. 4A-4C. (4A) The PL spectra of colloidal green-light-emittingphosphors before (solid lines) and after oxidation (dot lines). (4B) ThePL spectra of a mixture of colloidal red- andyellow-green-light-emitting phosphors, where the black- and grey-dotlines represent different mixing ratios. (4C) The light spectra from a405 nm InGaN LED with the red-light-emitting phosphor thin film as lightconverters, as a function of increasing LED intensity (from thebottom-dash line to the top-solid line).

FIGS. 5A-5D. (5A), (5C) and (5D) PL spectra of the SiQD-phosphors withhydroxyl-termination (grey dotted lines), TMPS-passivation (red, orangeand yellow lines) and APTES-passivation (green lines). (5B) Excitationspectra of the SiQD-phosphors in (a). CPS: counts per second.

FIGS. 6A and 6B. (6A) The surface chemistries of TMPS- andAPTES-passivation. (6B) A photograph of the SiQD-phosphors with TMPS-and APTES-passivation, under 365 nm UV excitation.

FIG. 7. Lighting spectra of 405 nm InGaN LEDs covered with only thered-emitting PDMS film (solid line), and with the red- and theyellow-emitting PDMS films (dotted line). Inset: a photograph of thePDMS films, under 365 nm UV excitation.

FIGS. 8A-8C are diagrammatic illustrations of embodiments of lightingdevices incorporating silicon phosphors in accordance with the disclosedembodiments.

DETAILED DESCRIPTION

Phosphors formed using silicon nanoparticles are provided. The phosphorsexhibit bright fluorescence and high quantum yield, making them idealfor lighting applications. Methods for making the silicon phosphors arealso provided, along with lighting devices that incorporate the siliconphosphors.

In one aspect, a silicon phosphor is provided. In one embodiment, thesilicon phosphor includes:

a core comprising a silicon particle;

a plurality of silicon nanoparticles attached to the core;

a silicon oxide layer substantially encapsulating each of the pluralityof silicon nanoparticles; and

a passivating layer comprising a plurality of passivating ligands boundto the silicon oxide layer.

The silicon phosphor is capable of photoluminescence. Particularly, thesilicon phosphor absorbs light in a particular wavelength range andemits light in a relatively lower-energy wavelength range. For example,in one embodiment, the silicon phosphor absorbs UV light and emitsvisible red light.

Phosphors have many uses in lighting applications. The provided siliconphosphors can be readily substituted for traditional phosphors usingmethods known to those of skill in the art.

The silicon phosphor may be better understood with reference to FIGS. 1Aand 1B. A silicon phosphor 100 is illustrated that comprises a core 102and a plurality of nanoparticles 104. The plurality of nanoparticles 104can be seen in detail in FIG. 1B, wherein each nanoparticle 104 includesa silicon nanoparticle 106 substantially encapsulated with an oxidelayer 108. The oxide layer 108 is passivated (i.e., coated) with aplurality of passivating ligands 110. Depending on the method by whichthe silicon phosphor 100 is fabricated, an oxide layer 112 may grow onsurface of the core 102 in addition to the silicon nanoparticle 106.Similarly, a layer of passivating ligands may also coat the surface ofthe optional oxide layer 112. However, such optional coatings areincidental to the functionality of the silicon phosphor 100, which usesthe oxide layer 108 and passivating ligands 110 to control the emissioncharacteristics of the nanoparticles 104. The core 102 does not haveemission characteristics and only serves to quench and/or scatter lightimpinging on its surface.

The core of the silicon phosphor is formed from a silicon particle. Thesilicon particle may be single-crystalline or polycrystalline. In oneembodiment, the core has a diameter of between about 10 nm and about 10μm.

Depending on the size of the core certain wavelengths of light may bescattered. Such scattering can be beneficial in lighting applicationswhere it is desirable to disperse light. In one embodiment, the core iscapable of scattering UV, visible, and infrared light. In oneembodiment, the core is only capable of scattering visible light.

A plurality of silicon nanoparticles are attached to the core. By“attached to the core” it meant that the silicon nanoparticles areimmobilized on the surface of the core. This may be by bonding,electrostatic, or other means known to those of skill in the art. Theattachment must be robust enough such that the silicon phosphors do notdissemble when in solution. The silicon nanoparticles aresingle-crystalline. Single-crystalline silicon nanoparticles produce thehighest photoluminescent quantum yield. In one embodiment, the pluralityof silicon nanoparticles each has a diameter between 1 nm and 5 nm.

The size of the silicon nanoparticles largely defines the absorption andemission qualities of the silicon phosphor. In this regard, the siliconnanoparticles are “quantum dots” (QDs), a term that is well known tothose of skill in the art.

In one embodiment, the plurality of silicon nanoparticles partiallycovers the core. In another embodiment, the plurality of siliconnanoparticles completely covers the core (i.e., forms a continuous layerof silicon nanoparticles in a shell around the core).

The silicon nanoparticles are each substantially encapsulated by asilicon oxide layer. The oxide layer is not an atmospheric oxide, but isinstead an intentionally-grown oxide formed in solution orhigh-temperature oxidation. The oxide layer serves to enhance thephotoluminescent qualities of the silicon phosphor. The oxide layer istypically 1 nanometer or greater in thickness.

As used herein the term “substantially encapsulated” means that thesilicon nanoparticle is encapsulated by the oxide on every surface thatis exposed to the environment surrounding the silicon nanoparticle.Depending on the attachment mechanism between the core and the siliconnanoparticle, a portion of the silicon nanoparticle may abut the coreand therefore would not be oxidized. Such an arrangement is illustratedin FIG. 1B.

The oxide layer of each silicon nanoparticle is covered by a passivatinglayer. The passivating layer is a plurality of passivating ligandscovalently bound to the oxide (e.g., via a siloxane bond). Thepassivating layer further enhances the photoluminescent qualities of thesilicon phosphor. The ligands are referred to as “passivating” becausethey protect the emitting silicon nanoparticles fromphotoluminescence-quenching molecules, such as water or ethanol, andenable stable suspension in the desired solvent systems.

In one embodiment, the passivating ligand is selected from the groupconsisting of alkyls, alkenyls, alkynyls, aromatics, aromaticheterocycles, conjugated aromatics, polyenes, cyanides, hydroxys,alkoxys, carboxylates, phenoxys, siloxys, cyanates, thioalkyls,thioaryls, thiocyanates, silylthios, substituted silyl groups, aminogroups, mono-substituted amines, di-substituted amines, imino groups,silylaminos, alkoxy silanes, alkyl alkoxysilanes, and aminoalkoxysilanes.

In one embodiment, the plurality of passivating ligands includestrimethoxypropylsilanes (TMPS). In one embodiment, the plurality ofpassivating ligands includes (3-aminopropyl)trimethoxysilanes (APTS).

In another aspect, a method of making silicon phosphors is provided. Inone embodiment, the method comprises the steps of:

(a) electrochemically etching silicon to provide a plurality of coresilicon particles each having a plurality of silicon nanoparticlesattached thereto;

(b) isotropically etching the plurality of silicon nanoparticles toprovide a plurality of etched silicon nanoparticles;

(c) capping the plurality of etched silicon nanoparticles with an oxidelayer; and

(d) passivating the oxide layer with surface ligands.

The method is illustrated as a flow chart in FIG. 1C.

In the first step 202, silicon is electrochemically etched to provide aplurality of core silicon particles each having a plurality of siliconnanoparticles attached thereto. Referring back to FIG. 1B, this stepessentially provides a core 102 with nanoparticles 106 attached to thesurface. This step can be accomplished using electrochemical etchingtechniques known to those of skill in the art. Further specific examplesare set forth in the EXAMPLES below. In one embodiment, the silicon iselectrochemically etched in a solution of HF and methanol. In oneembodiment, the silicon is a silicon wafer that is submerged anddisposed horizontally in the etching solution.

The silicon may be doped or undoped. In one embodiment, the silicon is ap-type silicon wafer. In another embodiment, the silicon is an n-typesilicon wafer.

When a silicon wafer is used for the electrochemical etching step, theplurality of silicon nanoparticles are obtained from the wafer surfaceby mechanically pulverizing the electrochemically etched silicon.

In the second step 204, the plurality of silicon nanoparticles isisotropically etched to provide a plurality of etched siliconnanoparticles. The etching is typically performed using a solutionconfigured to isotropically etch silicon. In one embodiment, theplurality of silicon nanoparticles are isotropically etched in anaqueous solution comprised of HNO₃ and HF.

The isotropic etch is performed at a controlled rate so as to reduce thesize of the silicon nanoparticles to a desired level and to homogenizethe surface states of the particles, which enhances photoluminescence.

In the third step 206, the plurality of etched silicon nanoparticles arecapped with an oxide layer. The oxide layer is grown using asolution-based technique, such as HNO₃ or other acids known to those ofskill in the art. The thickness of the oxide is greater than thethickness of an oxide grown atmospherically at room temperature (i.e., anative oxide). In one embodiment, the oxide is 1 nm or greater inthickness.

In the fourth step 208, the oxide-encapsulated silicon nanoparticles arepassivated with surface ligands. The surface ligands covalently bond tothe oxide and enhance photoluminescence.

In one embodiment, the passivating ligand is selected from the groupconsisting of alkyls, alkenyls, alkynyls, aromatics, aromaticheterocycles, conjugated aromatics, polyenes, cyanides, hydroxys,alkoxys, carboxylates, phenoxys, siloxys, cyanates, thioalkyls,thioaryls, thiocyanates, silylthios, substituted silyl groups, aminogroups, mono-substituted amines, di-substituted amines, imino groups,silylaminos, alkoxy silanes, alkyl alkoxysilanes, and aminoalkoxysilanes.

In one embodiment, the plurality of passivating ligands includestrimethoxypropylsilanes (TMPS). In one embodiment, the plurality ofpassivating ligands includes (3-aminopropyl)trimethoxysilanes (APTS).

In one embodiment, the method further comprises a step of separating thesilicon phosphors into nano-scale silicon phosphors and micron-scalesilicon phosphors. In a further embodiment, separating comprisescentrifuging a solution of the silicon phosphors such that themicron-scale silicon phosphors sediment and the nano-scale siliconphosphors remain in a supernatant, wherein the method further comprisesremoving the supernatant comprising the nano-scale silicon phosphors. Asdescribed in EXAMPLE 1 below, separating relatively “larger” (i.e., 100nm or greater in diameter) phosphors from “smaller” (i.e., 100 nm orless in diameter) phosphors can improve the photoluminescence of asolution containing only smaller phosphors.

In another aspect, a lighting device is provided that incorporates asilicon phosphor as described herein. The silicon phosphors can beutilized in any manner that conventional, known phosphors are used forlighting applications. Therefore, the silicon phosphors can be used withany compatible light source to provide a photoluminescent response. Inone embodiment, the lighting device is selected from the groupconsisting of a light-emitting diode and a fluorescent lamp.

In one embodiment, the silicon phosphor is applied directly to a lightsource. In such an embodiment, the silicon phosphor may be excited by aUV, blue, or other “higher energy” wavelength. An illustrativeembodiment is shown in FIG. 8A, in which a lighting element 300 includesa light source 304 (e.g., a LED) disposed on a substrate 302. As isknown to those of skill in the art, the substrate 302 may include areflective surface designed to direct light from the light source 304 ina particular direction. A layer of silicon phosphors 306, such as thoseprovided herein, are disposed directly on the surface of the lightsource 304. When the light source 304 is illuminated, the siliconphosphors 306 convert light photoluminescence to alter the spectraloutput of the lighting element 300 compared to a similar lightingelement with no silicon phosphors.

In one embodiment, the silicon phosphor is incorporated in asubstantially optically translucent matrix placed adjacent to a blue orUV emitting light source. Such an embodiment is illustrated in FIG. 8B,which illustrates a lighting element 310 having a light source 304 on asubstrate 302. A translucent matrix 312 is disposed contacting andsurrounding the light source 302, as is typically used to package LEDs.Within the translucent matrix 312, silicon phosphors 314 as disclosedherein are disposed. Optionally, other phosphors and/or light diffusers316 can be disposed in the translucent matrix 312 so as to further alterthe spectral output of the lighting element 310.

In one embodiment, the silicon phosphor is embedded on the surface of abulb which encapsulates a light source. Such an embodiment isillustrated in FIG. 8C, which illustrates a lighting element 320 havinga light source 304 on a substrate 302. A bulb 322 is disposedsurrounding, but not abutting, the light source 302, as is typicallyfound in bulb-based lighting (e.g., incandescent bulbs). Within the bulb322, silicon phosphors 324 as disclosed herein are disposed. Optionally,other phosphors and/or light diffusers 326 can be disposed in the bulb322 so as to further alter the spectral output of the lighting element320.

In any of the provided embodiments, the silicon phosphors may be part ofa lighting device that ultimately outputs white light via a combinationof the spectral output of the lighting element, the silicon phosphors,and (optionally) other phosphors.

In one embodiment, the lighting device (e.g., 300, 310, or 320) isselected from the group consisting of a light-emitting diode and afluorescent lamp.

The following examples are included for the purpose of illustrating, notlimiting, the described embodiments.

EXAMPLES Example 1 Silicon Quantum Dots Passivated withTrimethoxypropylsilane (TMPS)

Here we demonstrate low-cost, heavy-metal-free, air-stable andwavelength-tunable SiQD-based phosphor materials which are synthesizedby the electrochemical etching method. Although containing micro-sizeparticles, the colloidal red-light-emitting phosphors in chloroformexhibit PL external QE (EQE) of 15.9%. Furthermore, their thin films canbe efficiently excited by InGaN LEDs and are stable in room conditionowing to an oxide passivating shell. All these properties enable theSiQD-based phosphor materials a good candidate as light converters forlighting and display applications.

Experimental

We electrochemically etched p-type boron-doped Si wafers with (100)orientation and 5-20 ohm-cm resistivity in a mixture of HF and methanol.A typical etching condition was at a constant current density of 3.5mA/cm² for about an hour under stirring. After rinsing with isopropanoland drying in air, the wafer surface exhibited bright orangish/reddishPL and became highly hydrophobic as a result of complete hydridetermination (Si—H). By simply adjusting the electrolyte ratios, etchingcurrent density and etching time, the PL peak wavelength can vary from710 nm to 650 nm after the electrochemical etching. The luminescentpowders harvested from the wafer surface were then dispersed in ethanoluniformly with the help from ultra-sonication. For the synthesis ofnear-IR- to red-light-emitting phosphors, HNO₃ was slowly added to theSi powder suspension under stirring, resulting in a final concentrationof 20%. For the synthesis of orange-, yellow-, and green-light-emittingphosphors, both HNO₃ and diluted HF were added to yield a finalconcentration of 20% for HNO₃ and 0.5-1% for HF. During the isotropicetching, the PL intensity gradually increased and the PL colorcontinuously blue-shifted from red to orange, yellow, and green in thecase of HNO₃/HF etching. Once the desired PL wavelength was reached, thereaction was effectively stopped by taking the suspension tocentrifugation (4000 rpm for 20 minutes), decanting the supernatant andre-dispersing the precipitate in ethanol. The process can be repeatedseveral times to remove residual HNO₃ and HF.

For further functionalization with alkoxysilanes, the luminescentsilicon powders, treated as described above to obtain hydroxyltermination (Si—OH), were dispersed in ˜5 wt % trimethoxypropylsilane(TMPS) in ethanol, and then refluxed with stirring for about 12 hours.Subsequently, the suspension was taken to centrifugation (4000 rpm for20 minutes) and the precipitate can form a stable suspension innon-polar solvents, such as chloroform and hexanes. A typicalconcentration of 1 mg/ml was used for optical characterizations and 10mg/ml for thin film fabrication. The PL and excitation spectra of allliquid samples were measured by using a fluorometer (Jobin Yvon HoribaFluorolog FL-3) with xenon short arc lamp as light source. The PL EQE ofall liquid and solid-state thin film samples were measured by using anintegrating sphere system (Hamamatsu Absolute PL Quantum YieldMeasurement System).

Results and Discussion

The normalized PL and excitation spectra of colloidal SiQD-basedphosphors in ethanol are shown in FIGS. 2A and 2B, respectively. The PLof near-IR- (708 nm), red- (674 nm), orange- (624 nm), and yellow- (610nm) light-emitting samples were measured with 405 nm excitation, whilethe PL of yellow-green- (585 nm) and green- (549 nm) light-emittingsamples were measured with 365 nm excitation. Specifically, thered-light-emitting phosphors which were treated by HNO₃ have a broadbandexcitation spectrum, maintaining 70% of excitation efficiency from 345nm to 475 nm (the red-line in FIG. 2B). Similar results were alsoobserved for the near-IR-light-emitting phosphors. However, with HNO₃/HFetching, the orange-, yellow-, yellow-green-, and green-light-emittingphosphors can only be efficiently excited by a narrow band ofultra-violet (UV) light and the excitation spectra slightly blue-shiftfor the longer HNO₃/HF etching time. Note that several energy linesaround 475 nm in the excitation spectra likely originate from the xenonlamp.

The PL spectra and EQEs of the colloidal red-light-emitting phosphorsuspension with different kinds of surface passivation are shown in FIG.2C. The electrochemical etching resulted in bright orangish/reddish PLon the wafer surface. However, after the fluorescent powders weresuspended in ethanol, the PL intensity became significantly quenched.The PL quenching due to ethanol has also been observed inhydrogen-terminated porous silicon. The following HNO₃ treatmentessentially grew a high-quality oxide capping which isolates thePL-emitting SiQDs from the environment. Experimentally, the enhancementof PL intensity in ethanol was clearly observed for samples treated byeither HNO₃ only or a HNO₃/HF mixture. The PL EQE of red-, orange-,yellow-, and yellow-green-light-emitting phosphors in ethanol weremeasured to be 5.3%, 6.3%, 3.6% and 1.4%, respectively. (All sampleswere excited by 405 nm, except the yellow-green sample which was excitedby 365 nm.) To improve solubility in non-polar solvents, the phosphormaterials with hydroxyl termination reacted with alkoxysilanes (here weused TMPS) through the silanization reaction. In FIG. 2C, theTMPS-treated red-light-emitting phosphors in chloroform showed a muchimproved PL EQE of 15.9% (with 405 nm excitation). The improvementpossibly arises from the higher degree of oxide capping as a result ofreflux and the solvent environment changed from polar to non-polarsystems. The PL peak wavelength slightly red-shifted (from 656 nm to 674nm) from hydride to hydroxyl termination both in ethanol, while therewas almost no change of PL peak position from hydroxyl termination inethanol to alkyl termination in chloroform, as shown in FIG. 2C.Although containing a certain amount of micro-size silicon particles,the PL EQE (15.9%) achieved here is comparable to a recent result inliterature (SiQD-ensemble PL QE of 20%). However, their synthesisrequired multiple sedimentation processes and the capping structuresinvolved complex organic residues. To further improve the PL EQE, anefficient separation technique to harvest SiQDs from the wafer with highproduction yield has been investigated. After extended ultra-sonicationand centrifugation, we obtained a clear red-light-emitting suspensioncontaining only nano-size particles in ethanol, and the PL EQE of thatsample was measured to be 9.4%, compared to 5.3% with both nano- andmicro-size particles in ethanol.

The red-light-emitting phosphors in non-polar solvents can be processedinto thin films by drop-casting or spin-coating. FIGS. 3A and 3B showthe scanning electron microscope (SEM) images of the thin film in lowand high magnification, respectively. For the micro-size siliconparticles, they are mostly of sizes ranging from 1 to 5 μm. Somesubmicron-size clusters can also be found among the gaps betweenparticles. In high magnification, nano-porous surfaces which are mostlycomposed of PL-emitting SiQDs can be found on top of each micro-sizesilicon particle. FIG. 3C shows “UW” painted a glass slide by using thered-light-emitting phosphors suspending in hexanes/octanes. Thephotograph was taken under illumination from a 365 nm UV lamp.

For applications in lighting and displays, the air-stability of allphosphor materials has been an important requirement. To verify suchquality, the PL EQE of the red-light-emitting phosphor thin film, suchas the one in FIG. 3C, was initially measured to be 13.2% (with 405 nmexcitation), and it maintained more than 10% for the following two weekswhen the thin film was stored in room condition. Meanwhile, the PL peakposition also remained unchanged. This high stability likely arisingfrom the oxide passivation was also found in near-IR-, orange-, andyellow-light-emitting samples. For yellow-green- andgreen-light-emitting phosphors whose PL peak at <590 nm, although cappedwith an oxide passivating shell, their PL peak wavelengths still slowlyred-shifted toward yellow as exposed in air. This phenomenon is likelyattributed to the electron- and hole-trap-states resulting from Si═Obonds which form localized energy levels in the bandgap of SiQDs andtherefore red-shift the PL peak to around 590 nm. For example, in FIG.4A, the PL peak of a colloidal phosphor suspension in ethanol(yellow-green-solid line) red-shifted from its original 583 nm to 590 nmupon oxidation by mixing with H₂O₂ (yellow-green-dot line). Noticeably,the PL peak intensity dropped about 30% at the same time. Forcomparison, the same colloidal phosphor suspension was first treatedwith diluted HF to remove the surface oxide passivation, thenre-dispersed in ethanol (green-solid line), and mixed with H₂O₂ at theend (green-dot line). Upon HF treatment, the PL peak wavelengthblue-shifted from its original 583 nm to 533 nm as a result of decreasedQD size by HF etching, while the PL peak intensity also dropped about70% likely due to ethanol quenching on the hydride-terminated surface.After adding H₂O₂, the PL dramatically disappeared due to the formationof efficient non-radiative channels. Similar results have been found inthe previous study of porous silicon that poorly oxidized surface leadsto catastrophic loss of PL, while high-quality oxide passivation formedchemically or thermally can enhance PL efficiency.

White light generation has drawn much attention owing to the expandingmarket of general lighting and backlight for liquid-crystal displays(LCDs). Mixing the light from a blue InGaN LED and that of yellowphosphors (cerium-doped yttrium aluminum garnet (YAG)) has been the mostefficient and cost-effective way for obtaining a white LED. However, theresulting pale white light cannot display color faithfully due to arelatively small color gamut. As shown in FIG. 4B, here we demonstratemixing colloidal yellow-green- (585 nm) and red- (677 nm) light-emittingSiQD-based phosphors to yield a yellow-to-orange broadband emission(full-width-at-half-maximum (FWHM)=181 nm). By mixing with more colorsespecially in the blue to green wavelengths, the phosphor materials canpotentially generate continuous emission covering the whole visiblespectrum for the general lighting application. Furthermore, todemonstrate the integration with conventional LEDs, FIG. 4C shows thelight spectra from a 405 nm InGaN LED (purchased from Thorlabs, Inc.)with the red-light-emitting phosphor thin film as light converters, as afunction of increasing LED intensity. Similar demonstration of usingCdSe/ZnSe QDs as light converters in comparison with conventional YAGphosphors was found in literature. To achieve a white LED consisting ofa blue LED with red and green SiQD-based phosphors as three primarycolors, the low excitation efficiency of green phosphors in bluewavelengths is now the main obstacle, as shown in FIG. 2B. The studiesof green phosphors of higher blue excitation efficiency and betterpassivation to prevent PL red-shifting due to Si═O trap-states arecurrently in progress.

To determine the effect of micron-size Si particles in quenching PLquantum yield (QY), we took a suspension comprising both micron-sizedand nano-sized silicon quantum dot phosphors passivated with TMPS tocentrifugation and collected the supernatant. Under room lighting, thephosphor suspension before centrifugation looked “muddy” because themicron-size Si particles enhance light scattering and trapping insidethe solution. The phosphor suspension after centrifugation becameoptically transparent with slightly yellowish color. Under 365-nm UVexcitation, both samples showed bright red-emitting PL and the same PLspectrum. However, the PL QY of the sample after centrifugationincreased to 37%, compared to 16% before centrifugation.

Conclusion

In summary, we have demonstrated colloidal SiQD-based phosphor materialswhich emitted bright PL from near-IR to green. The phosphors were firstsynthesized by the electrochemical etching method. The subsequent HNO₃or HNO₃/HF isotropic etching not only controlled the QD size so as tothe PL emission due to quantum confinement effect, but also capped thematerial surface with a high-quality oxide passivating shell. Thered-light-emitting phosphors with HNO₃ treatment can be efficientlyexcited by blue light, while the other phosphors with HNO₃/HF treatmentcan be excited by a relatively narrow band in UV. The phosphor materialswith hydroxyl termination can further react with alkoxysilanes to becomestable suspensions in non-polar solvents. The TMPS-modifiedred-light-emitting phosphors in chloroform showed PL EQE of 15.9%, andtheir thin films are stable in room condition (PL EQE equal to 13.2%initially and kept more than 10% for two weeks). The SiQD-based phosphormaterials presented here have shown a great potential as low-cost,heavy-metal-free, air-stable, and wavelength-tunable light-convertersfor applications in general lighting and display backlight systems.

Example 2 Silicon Quantum Dots Passivated with(3-aminopropyl)triethoxysilane (APTES)

Semiconductor quantum dots (QDs) with wide-range wavelength-tunability,high photoluminescence (PL) quantum yield (PLQY) and narrow emissionline-width have been considered as a potential replacement forrare-earth-element (REE) phosphors used in white light-emitting-diodes(LEDs). Using QDs as phosphors for lighting not only improves the colorrendering performance but also lessens the demand for REEs and hence theenvironmental degradation associated with their extraction and refining.However, previous QD-phosphor research was predominately based on II-VIcompound semiconductor nanocrystals. Although their exquisite core/shellstructures can lead to high PLQY, the heavy-metal-toxicity from cadmiumions might limit their potential for wide-spread commercialization.

Previously, we demonstrated air-stable and non-toxic silicon quantum dotphosphors (SiQD-phosphors) with PL wavelengths tunable by a HNO₃/HFisotropic etching process which decreases the SiQD sizes. The longeretching time resulted in smaller dot sizes and thus the more PLblue-shift due to quantum confinement effect. Here, we demonstrated thatin addition to the SiQD sizes, the PL properties (peak wavelength andintensity) are closely dependent on the surface passivation chemistriesof SiQDs. Efficient PL with versatile colors can be achieved byeffectively controlling these two factors. The red- and yellow-emittingSiQD-phosphors can be efficiently excited by 405 nm InGaN LEDs forpotential general lighting applications. The whole synthesis process isperformed under ambient conditions, using common chemicals. In contrast,other main strategies for SiQD synthesis, such as solution-basedprecursor reduction and aerosol decomposition of silane, inevitablyrequire critical conditions, special equipment or complex reactions.

We synthesized the SiQD-phosphors by starting with electrochemicaletching on a p-type Si wafer in a mixture of HF and methanol. A typicaletching condition was at a constant current density of 3.5 mA/cm² for 60minutes. After the electrochemical etching, we obtained free-standing Sipowders which exhibited weak red-emitting PL from the wafer substrateusing a mechanical pulverization process, and dispersed the Si powdersin ethanol. The Si powder suspension was then added with an aqueousmixture of HNO₃ and HF for an isotropic etching reaction. The PL colorslowly and continuously shifted from red to yellow as the etchingprocess continued. After the isotropic etching, all the Si powdersuspensions were treated with 20% HNO₃ to grow an oxide capping layer(hydroxyl-termination, Si—OH). The silicon oxide shell resulting fromthe etching process isolated the SiQDs from the PL quenching ethanol andthus increased the PL intensity.

For functionalization with alkyl-alkoxysilanes, the luminescent Sipowders with hydroxyl-termination were treated with 5 wt %trimethoxypropylsilane (TMPS) and re-dispersed in chloroform. Forfunctionalization with amino-alkoxysilanes, the Si powders were treatedwith 5 wt % (3-aminopropyl)triethoxysilane (APTES) and re-dispersed inwater. Each surface treatment step was effectively terminated byhigh-speed centrifugation, decantation of supernatant and re-dispersionin the desired solvents. A concentration of 1 mg of the SiQD-phosphorsin 1 mL of solvent (chloroform or water) was used for all opticalcharacterizations.

The PL and excitation spectra of the SiQD-phosphors withhydroxyl-termination, TMPS-passivation and APTES-passivation weremeasured by using a fluorometer (Jobin Yvon Horiba Fluorolog FL-3). Allthe PL spectra were acquired by using 405 nm excitation. By HNO₃/HFisotropic etching and subsequent HNO₃ treatment, we first obtained thehydroxyl-terminated SiQD-phosphor suspensions in ethanol with PL peakwavelengths at red (676 nm), orange (630 nm) and yellow (616 nm), asrepresented by grey dashed lines in FIGS. 5A, 5C and 5D, respectively.Then, half of each hydroxyl-terminated sample was put throughfunctionalization with TMPS, while the other half was treated withAPTES. The TMPS-passivated SiQD-phosphor suspensions in chloroform,represented by red, orange and yellow lines in FIGS. 5A, 5C and 5D, keptalmost the same PL peak wavelengths and intensities as thehydroxyl-terminated ones. In contrast, the APTES-passivatedSiQD-phosphor suspensions in water, represented by green lines in FIGS.5A, 5C and 5D, showed significant PL blue-shift in different amounts:from 676 nm to 595 nm, 630 nm to 593 nm and 616 nm to 577 nm for thered, orange and yellow samples, respectively. At the same time, the PLintensity became lower than the hydroxyl-terminated ones, and moreintensity decrease was observed for samples with shorter startingwavelengths. The decrease of PL intensity is likely due to the strongpolarity nature of the passivating ligands (APTES) and the solventenvironment (water). The dipoles attract either electrons or holes tosurface trap states and therefore reduce the PL intensity.

As shown in FIG. 5B, the TMPS-passivated (red line) and thehydroxyl-terminated (grey dashed line) SiQD-phosphors showed broadbandexcitation spectra, maintaining excitation efficiency of more than 70%from 350 nm to 473 nm. In comparison, the APTES-passivatedSiQD-phosphors (green line) showed a narrower excitation band width,however still maintaining about 70% of excitation efficiency at 405 nm.This property ensures the red- (red line) and the yellow-emitting (greenline) phosphors in FIG. 5A can be excited efficiently by conventional405 nm InGaN LEDs.

From the scanning electron microscope (SEM) images, the phosphors arecomposed of micron-size Si particles (diameters ranging from around 1 to5 μm) with nano-porous surfaces. The visible photoluminescent SiQDs(diameter less than around 3.5 nm) are embedded in the porous Si layeras nano-size columnar structures. The surface chemistries of TMPS- andAPTES-passivated SiQD-phosphors are shown in FIG. 6A. Both TMPS andAPTES have three carbons in the hydrocarbon chains, while APTES isterminated with an additional amino group at the end which gives APTES amuch higher polarity than TMPS. We used Fourier transform infraredspectroscopy in attenuated total reflectance mode (FTIR-ATR) todetermine the surface compositions of the SiQD-phosphors in dry powderforms. Both TMPS- and APTES-passivated samples showed similar spectra.The most prominent peak was at 1050 to 1100 cm⁻¹ which can be a jointcontribution from Si—O—Si and Si—C bonds of the attached TMPS or APTESligands. A smaller peak was observed at 2350 cm⁻¹ which is likelyattributed to either O—Si—H or Si—H bonds. Although the Si surface hadbeen treated with 20% HNO₃, some hydride groups were still not oxidizedto hydroxyl groups.

The red-emitting TMPS-passivated SiQD-phosphors in chloroform and theyellow-emitting APTES-passivated SiQD-phosphors in water are shown inFIG. 6B, under 365 nm ultra-violet (UV) excitation. Their correspondingPL spectra can be found in FIG. 5A. Note that the phosphors under 365 nmexcitation exhibit almost the same PL spectra as under 405 nmexcitation. The absolute PLQY of both liquid samples were measured byusing an integrating sphere system (Hamamatsu Absolute PL Quantum YieldMeasurement System). The PLQY=17% and 4% for the TMPS- andAPTES-passivated samples, respectively. The red-emitting phosphors haveshown PLQY close to previously reported SiQDs with visible PLQY=20 to30% and synthesized by more complex methods. It is worth noticing thatduring the PLQY measurements, a significant portion of the PL photonsemitted by the SiQDs were re-absorbed by the micron-size Si particles inthe integrating sphere and hence were not collected by the spectrometer,and cannot contribute to the PLQY tally. Therefore, to improve PLQY, wewill need to increase the production yield of free-standing SiQDs anddecrease the amount of micron-structures in the phosphors.

After being embedded in a polymer matrix, such as polydimethylsiloxane(PDMS) used in this work, the SiQD-phosphors showed consistent PL colorsas they have in solution or powder forms. The inset of FIG. 7 shows thePDMS thin films embedded with the red-emitting TMPS-passivated and theyellow-emitting APTES-passivated SiQD-phosphors, under 365 nm UVexcitation. We placed the film in front of 405 nm InGaN LEDs andcharacterized the spectra of light passing through the film, as shown inFIG. 7. With only the red-emitting PDMS film (solid line), the spectrumwas composed of one blue (405 nm) and one red (674 nm) peak, resultingin a purple color. With the red-emitting PDMS film stacking on top ofthe yellow-emitting one (dotted line), the peak was shifted to yellow(608 nm) and was broadened, i.e., full width at half maximum (FWHM)increasing from 142 nm to 153 nm, and the integrated PL intensity becamealmost half. To cover the whole visible spectrum, we are currentlydeveloping green-emitting SiQD-phosphors with PL peak wavelengths ataround 500 nm. However, blue- or green-emitting SiQDs with enlarged bandgap, due to quantum confinement effect, suffer from surface trappingstates associated with Si═O bonds which limit the radiativerecombination energy to around 590 nm. Therefore, rather than oxidecapping, an oxygen-free surface passivation method, such ashydrosilylation with alkenes or alkynes, will be preferred for blue- orgreen-emitting SiQDs.

APTES is only different from TMPS by having an additional —NH₂termination at one end, as shown in FIG. 6C. However, while there isalmost no change of PL peak wavelength with TMPS-passivation, theAPTES-passivated SiQD-phosphors showed significant PL blue-shiftcompared to the hydroxyl-terminated ones. It is likely that the muchstronger polarity of APTES changes the charge distribution within theSiQDs and thus affects their PL spectra. Similarly, computationalstudies have shown that covering SiQDs with alkyl-groups (Si—C) resultedin minimal changes in PL spectra. In contrast, SiQDs with —HN₂, —SH and—OH terminations exhibited obvious PL changes compared to hydride-cappedSiQDs. Experimentally, 1 to 2 nm SiQDs capped with either allylamine(CH₂═CH—CH₂—HN₂) or 1-heptene (CH₂═CH—(CH₂)₄—CH₃) have also shownremarkably different PL properties. Similar to our case, both ligandshave short alkyl-chains, but allylamine has an —HN₂ termination at oneend.

By using a time-correlated single photon counting (TCSPC) technique with375 nm laser pulses repeated in more than 1 μs, we found that both TMPS-and APTES-passivated SiQD-phosphors have PL decay in the microsecondsrange. Such slow PL, compared to fast PL in the nanoseconds range ofdirect band gap CdSe QDs, are likely due to ultra-fast trapping into,and then slow decay from, the oxide surface states which form within theband gap during oxidation. In our SiQD-phosphors, the silicon oxidecapping layer resulting from the HNO₃ treatment should be responsiblefor the air-stable and long-lifetime PL.

In conclusion, the SiQD-phosphors with hydroxyl-termination weresynthesized by electrochemical etching, followed by HNO₃/HF isotropicetching for controlling the PL peak wavelength and HNO₃ treatment forcapping with an oxide shell. Subsequent functionalization with APTESinduced significant PL blue-shift. In contrast, functionalization withTMPS resulted in almost no change of PL peak wavelength. All theSiQD-phosphors have shown air-stable and consistent PL in solution orpowdered forms, or after being embedded in a polymer matrix. Thered-emitting TMPS-passivated phosphors (PLQY=17%) and theyellow-emitting APTES-passivated phosphors (PLQY=4%) can be efficientlyexcited by 405 nm InGaN LEDs for potential applications in generallighting.

While illustrative embodiments have been illustrated and described, itwill be appreciated that various changes can be made therein withoutdeparting from the spirit and scope of the invention.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. A silicon phosphorcomprising: a core comprising a silicon particle; a plurality of siliconnanoparticles attached to the core; a silicon oxide layer substantiallyencapsulating each of the plurality of silicon nanoparticles; and apassivating layer comprising a plurality of passivating ligands bound tothe silicon oxide layer.
 2. The silicon phosphor of claim 1, wherein thecore has a diameter of between about 10 nm and about 10 μm.
 3. Thesilicon phosphor of claim 1, wherein the core is capable of scatteringUV, visible, and infrared light.
 4. The silicon phosphor of claim 1,wherein each nanoparticle in the plurality of silicon nanoparticles hasa diameter between 1 nm and 5 nm.
 5. The silicon phosphor of claim 1,wherein the plurality of passivating ligands are selected from the groupconsisting of alkyls, alkenyls, alkynyls, aromatics, aromaticheterocycles, conjugated aromatics, polyenes, cyanides, hydroxys,alkoxys, carboxylates, phenoxys, siloxys, cyanates, thioalkyls,thioaryls, thiocyanates, silylthios, substituted silyl groups, aminogroups, mono-substituted amines, di-substituted amines, imino groups,silylaminos, alkoxy silanes, alkyl alkoxysilanes, and aminoalkoxysilanes.
 6. The silicon phosphor of claim 1, wherein the pluralityof passivating ligands includes trimethoxypropylsilanes.
 7. The siliconphosphor of claim 1, wherein the plurality of passivating ligandsincludes (3-aminopropyl)trimethoxysilanes.
 8. A method of making siliconphosphors comprising: (a) electrochemically etching silicon to provide aplurality of core silicon particles each having a plurality of siliconnanoparticles attached thereto; (b) isotropically etching the pluralityof silicon nanoparticles to provide a plurality of etched siliconnanoparticles; (c) capping the plurality of etched silicon nanoparticleswith an oxide layer; and (d) passivating the oxide layer with surfaceligands.
 9. The method of claim 8, wherein the silicon iselectrochemically etched in a solution of HF and methanol.
 10. Themethod of claim 8, wherein the silicon is a p-type silicon wafer. 11.The method of claim 10, wherein the plurality of silicon nanoparticlesare obtained from the wafer surface by mechanically pulverizing theelectrochemically etched silicon.
 12. The method of claim 8, wherein theplurality of silicon nanoparticles are isotropically etched in anaqueous solution comprised of HNO₃ and HF.
 13. The method of claim 8,wherein the plurality of etched silicon nanoparticles are capped with asilicon oxide layer in a solution comprising HNO₃.
 14. The method ofclaim 8, wherein the surface ligands are selected from alkyls, alkenyls,alkynyls, aromatics, aromatic heterocycles, conjugated aromatics,polyenes, cyanides, hydroxys, alkoxys, carboxylates, phenoxys, siloxys,cyanates, thioalkyls, thioaryls, thiocyanates, silylthios, substitutedsilyl groups, amino groups, mono-substituted amines, di-substitutedamines, imino groups, silylaminos, alkoxy silanes, alkyl alkoxysilanesand amino alkoxysilanes.
 15. The method of claim 8, wherein theplurality of surface ligands includes trimethoxypropylsilane.
 16. Themethod of claim 8, wherein the plurality of surface ligands includes(3-aminopropyl)trimethoxysilane.
 17. The method of claim 8 furthercomprising the step of separating the silicon phosphors into nano-scalesilicon phosphors and micron-scale silicon phosphors.
 18. The method ofclaim 17, wherein separating comprises centrifuging a solution of thesilicon phosphors such that the micron-scale silicon phosphors sedimentand the nano-scale silicon phosphors remain in a supernatant, whereinthe method further comprises collecting the supernatant comprising thenano-scale silicon phosphors.
 19. A lighting device comprising a siliconphosphor of claim
 1. 20. The lighting device of claim 19, wherein thesilicon phosphor is applied directly to a blue or UV emitting lightsource.
 21. The lighting device of claim 19, wherein the siliconphosphor is incorporated in a substantially optically translucent matrixplaced adjacent to a blue or UV emitting light source.
 22. The lightingdevice of claim 19, wherein the silicon phosphor is embedded within abulb which encapsulates a blue or UV emitting light source.
 23. Thelighting device of claim 19, wherein the lighting device is selectedfrom the group consisting of a light-emitting diode and a fluorescentlamp.